In general, a parallel robot of the SCARA type is understood to be a robot having at least two elements, each formed of an arm and a forearm, which are mounted on two independent supports, that are rotationally movable about an axis of rotation, which theoretically coincide, the respective ends of the two forearms being joined in articulated fashion, so that these two elements thus form a closed loop, each forearm being rotationally movable at least about a geometrical axis relative to the respective arm. It should be taken into consideration that the arm is attached to its support; these two elements together form one and the same rotationally movable part, and in certain cases, may essentially coincide.
In general, the outer ends of the two forearms bear an endpiece having a tool or any sensor. This endpiece is able to move in a plane perpendicular to the axis of rotation of the two supports. The angular position of the endpiece is modified by an identical movement of the two supports, and the distance between the endpiece and the axis of rotation (the radial position of the endpiece) is modified by a differential movement of the two supports.
In one variant, the endpiece is able to be displaced along the direction of the axis of rotation of the two supports, i.e., of the two arms mentioned above. In a further variant, the tool or the sensor is able to undergo its own rotary motion with the aid of a motor or an actuator, which is disposed in the endpiece.
In a simplified arrangement of a parallel robot of the SCARA type, this robot may therefore include only two articulated elements which are mounted in a manner allowing rotation about a body or a base of this robot, the joints between the arms and the forearms, as well as the joint which is assigned to the endpiece, being formed by pivots having axes which are parallel to each other and are parallel to the geometrical axis of rotation of the two movable supports, that in each case bear the two articulated elements in a specific plane.
U.S. Pat. No. 7,331,750 describes a parallel robot of the SCARA type having a relatively complex design. This robot is designed with a third articulated element, which makes it possible to move the endpiece along a direction that is essentially parallel to the axis of rotation of the two arms of the first and of the second articulated element. Thus, these first and second articulated elements each have a joint between their arm and forearm which is formed by a spherical joint.
The arms of the first and of the second articulated element of the robot in U.S. Pat. No. 7,331,750 are attached to two annular supports, which in each case form the two rotors of two circular motors supported one upon the other. Each of these two articulated elements is therefore assigned to an individual circular motor, the two circular motors being disposed one upon the other. Firstly, it should be noted that the first and the second articulated element do not have a symmetrical structure, since the annular supports of the two respective arms lie at different levels. The production of such a robot thus represents various assembly problems with effects on the functioning of this robot. Namely, it is difficult to perfectly align the two axes of rotation of the two annular rotors, which bear the first and the second articulated element, respectively. Generally, this problem of the alignment of the axes of rotation engenders a lack of precision in the control of the first and second articulated elements, which causes a displacement of the endpiece outside of the plane perpendicular to the axis of rotation. In addition, in the special case of the simple variant described before, in which the joints are formed by pivots, an alignment error of the axes of rotation of the two rotors produces stresses in these pivots, causing them great wear. It should also be considered that the conventional robot described above includes three independent motors, in order to bring about translations of the endpiece in a cylindrical space. Ultimately, this robot has a complex and costly construction.